Warbling vireos are smaller vireos, measuring 14 cm in length, 21.6 cm from wingtip to wingtip and weighing in at 12 g. They are overall olive-gray above, with a gray crown that contrasts only slightly with their olive-gray backs. Like many vireos they feature a white supercilium and gray eyestripe. The eyestripe and lores for this species are a pale gray which gives them a "blank-faced" look that distinguishes them from other, more boldly patterned vireos. The flanks and sides are a pale yellow, while the throat, breast and belly are nearly white. Beaks and legs are dark gray to black in color. Males, females, and juveniles of this species all look alike. (Sibley, 2000)
Warbling vireos inhabit most of North and Central America. This species breeds across nearly the entire United States, excluding the southeast region. The breeding range reaches north to include most of southwest Canada. Few populations breed in Mexico but are restricted to the mountainous, Sierre Madre Occidental region. Warbling vireos are migratory birds that overwinter in Central America from Mexico to the northern edges of Nicaragua. Warbling vireos breed across the entire state of Michigan, and can be found from late spring to summer. (Gardali and Ballard, 2000)
Warbling vireos prefer to breed in deciduous or mixed deciduous-coniferous woodlands with adequate canopy openings. Forested riversides and thickets are also potential nesting habitats. They occasionally inhabit young, re-growing forests. They may also be found in urban parks, gardens, orchards or hedgerows. During the non-breeding season, warbling vireos inhabit a wider range of habitats including plantations and oak forests. They are common in shade-grown coffee plantations which retain native canopy trees and shrubs. During migration, common stopover sites include deciduous forest, shrubby habitats, and scrub forests in the southwest. Throughout all seasons, warbling vireos avoid boreal or pine dominated habitats. (Ehrlich, et al., 1988; Gardali and Ballard, 2000; James, 1976)
Warbling vireos are monogamous, meaning that one male and one female form a pair each season to breed and raise their young. It is unknown whether the same pairs form each year, or if birds return to the same site year after year. Pair formation likely occurs during migration, as most pairs have already formed by the time they arrive on the breeding grounds. Courtship displays generally begin with males engaging females in a chase flight. Afterward, the male may give courtship calls while fanning his tail and moving his body from side to side, facing the female. Females respond with wing-quivering, and when the male approaches she will strike her bill against his. Some mate feeding has been observed during migration as well. Once pairs have formed, the two individuals will both sing courtship calls while constructing the nest together. No reports of mate defense currently exist. (Ehrlich, et al., 1988; Gardali and Ballard, 2000)
In migratory populations, most warbling vireos arrive on the breeding grounds from mid-April to Mid-May and most have already formed pairs. Nest construction begins 2 to 7 days after arrival or pair formation on the breeding grounds. Nests are built by both males and females (though more-so by females) and are typically located high in the canopy, but height can range from 1 to 37 m. Like most vireos, they form a deep, hanging cup secured in a forked branch. Construction lasts 6 to 7 days and pairs incorporate leaves, grass, bark strips, pine needles, feathers or hair into the nest. Females lay an average clutch size of 4, white eggs which are spotted with brown or black. Eggs measure 19 mm in length. Incubation lasts 12 days on average, and the young fledge (leave the nest) after 13 to 14 days. Parents continue to feed their fledglings for at least 2 weeks, but exact independence date is unknown. Age at reproductive maturity is unknown but is presumed to be approximately 10 months or during an individual's first spring. (Ehrlich, et al., 1988; Gardali and Ballard, 2000)
Female warbling vireos select a suitable nesting site and perform most of the nest construction. Once the nest is completed and eggs have been laid, both males and females take turns incubating the clutch though females perform most of the incubation as well. Upon hatching, the young are altricial meaning that they lack feathers, are immobile and cannot open their eyes. The helpless young require constant feeding and brooding provided by both parents, although females more-so than males. Parents take turns watching over the nestlings and foraging for food, making sure that one parent is tending the brood at all times. Both parents remove fecal sacs from the nest which likely reduces risk of disease or predation. Once the hatchlings fledge (leave the nest), both parents continue to feed and care for the young for an additional two weeks. (Ehrlich, et al., 1988; Gardali and Ballard, 2000)
The longest-lived warbling vireo was an adult banded in California and recaptured 13 years later. Adult annual survivorship estimates range from 50 to 83%. Exact causes of mortality are unknown but may include brood parasitism by brown-headed cowbirds, nest depredation, or decline in habitat quality. (Gardali and Ballard, 2000; Gardali, et al., 2000; Ortega and Ortega, 2003)
Warbling vireos are long- or short- distance migrants that can potentially travel from the southern Northwest Territories of Canada to the northern tip of Nicaragua. Some populations in the mountainous regions of Mexico remain in the region year-round. They are primarily a diurnal species that is most active at dawn and dusk, but are also nocturnal during migration. Most of the year during the non-breeding and migratory seasons, this species is social and may be found in mixed-species flocks with an average of 10 other species. During the breeding season pairs form and become solitary and territorial against any intruders. Warbling vireos are primarily an arboreal species that forages and nests high in the canopy, on the peripheral edges. (Gardali and Ballard, 2000; James, 1976)
Territory size ranges from 1.2 to 3 hectares. Size is likely influenced by population density and habitat quality. Warbling vireos defend their territories but do not often use physical contact to deter intruders. They have been observed tolerating red-eyed vireos and yellow-throated vireos that sing within their territories. (Gardali and Ballard, 2000; James, 1976)
Like all birds, warbling vireos perceive their environment through the senses of sight, sound, smell, taste and touch. Vocal communications include male territorial song, courtship call, and a variety of contact, begging, and warning calls. The typical song is mnemonically described as "If I see you, I will seize you, and I'll squeeze you 'til you squirt!". Compared to other vireos, this call is undulating and more connected with an overall warbling quality. Calls are used between mates to locate each other, as well as warn of nearby predators. Pairs also use body postures to communicate during courtship. Male courtship begins with an aerial chase of the female which is followed by a stationary interaction where the male fans his tail and turns his body back and forth. The female responds with wing-quivering and will eventually peck at the male's beak when he approaches. (Gardali and Ballard, 2000)
Warbling vireos are primarily insectivores but will also consume spiders and berries in the fall and winter. They utilize a hover and glean feeding strategy, and capture nearly all of their food from peripheral leaves of trees or shrubs. Prey items include caterpillars and pupae of butterflies and moths, ladybug beetles, beetles, as well as spiders. Non-insect items consumed include elderberries and poison oak berries. These birds obtain water from small streams or from moisture built up on leaves. (Ehrlich, et al., 1988; Gardali and Ballard, 2000; James, 1976)
Currently there have been no observations of adult or nest predation, though it is known to occur. Certain bird species are heavily mobbed by warbling vireos and are presumed to be predators. These species include Steller’s jays, western scrub-jays, blue jays and common grackles. Western mammalian predators include red squirrels and western gray squirrels. Their dull, olive-gray coloration likely serves as camouflage in the tree canopy. (Gardali and Ballard, 2000)
Warbling vireos are primarily insectivores that likely impact local prey populations. They are common hosts of brood parasitic brown-headed cowbirds and have not evolved any method to remove or destroy the foreign eggs. During fall and winter, these birds include berries in their diets, and may serve a small role as a local seed disperser. One warbling vireo has been observed to have feather mites. (Ehrlich, et al., 1988; Gardali and Ballard, 2000; Ortega and Ortega, 2003)
There are no known adverse effects of warbling vireos on humans.
Warbling vireos are primarily insectivores, which may serve to reduce pest populations. They also sing a very pleasant song, and many bird watchers enjoy seeing and hearing them. (Gardali and Ballard, 2000)
Warbling vireos are of least concern to the IUCN Red List as the species has a large population size dispersed across a wide geographic range. As migratory birds, they are protected under the United States Migratory Bird Act. This species prefers forested habitats with significant portions of canopy openings and may thrive as a result of careful selective harvesting by the logging industry. There is a minor concern with regards to the effect of brown-headed cowbird brood parasitism. Warbling vireos have not yet evolved a method to identify, remove or destroy cowbird eggs which results in low productivity and may cause future population declines. Another concern is pesticide application, as warbling vireo populations may become locally extinct after foraging and nesting trees are sprayed. Warbling vireos are prevalent across Michigan and can be found in suitable habitats during the spring and summer. (Ehrlich, et al., 1988; Gardali and Ballard, 2000)
Rachelle Sterling (author), Special Projects, Tanya Dewey (editor), University of Michigan-Ann Arbor.
Ehrlich, P., D. Dobkin, D. Wheye. 1988. The Birder's Handbook: A Field Guide to the Natural History of North American Birds. New York, New York: Simon and Schuster.
Gardali, T., G. Ballard. 2000. "The Birds of North America Online" (On-line). Vireo gilvus. Accessed March 28, 2011 at http://bna.birds.cornell.edu/bna/species/551 doi:10.2173/bna.551.
Gardali, T., G. Ballard, N. Nur, G. Geupel. 2000. Demography of a declining population of warbling vireos in coastal California. The Condor, 102: 601-609. Accessed March 29, 2011 at http://www.prbo.org/cms/docs/terre/Gardali%20et%20al.%202000.pdf.
James, R. 1976. Foraging Behavior and Habitat Selection of Three Species of Vireos in Southern Ontario. The Wilson Bulletin, 88/1: 62-75.
Ortega, C., J. Ortega. 2003. Brown-headed cowbird (Molothrus ater) parasitism on warbling vireos (Vireo gilvus) in southwest Colorado. The Auk, 120/3: 759-764. Accessed March 29, 2011 at http://www.colostate.edu/depts/sjbrc/pubs/BrownHeadedCowbirdsParasitism.pdf.
Sibley, D. 2000. The Sibley Guide to Birds. New York: Alfred A. Knopf, Inc..